Ion mobility spectrometry (IMS) is an analytical technique that is commonly used for the detection of volatile and semi-volatile substances based on the mobility (drift time or reduced mobility) of ionized molecules of the substance in a gas inside an electric field. IMS can be suitable for applications including detection of illicit substances such as traces of explosives, narcotics or chemical warfare agents.
In a detection instrument using IMS, a drift region of a drift tube is an area between an ion shutter (or gate) and a detector. A conventional stacked drift tube is composed of a series of alternating conductive and insulating rings (e.g., where thickness varies with different designs) stacked together to form a tube of any desirable length. In the drift region, different ion species are subjected to a weak electrical field, causing them to move toward a detector. A drift gas is introduced into the drift region and made to flow opposite to the direction of ion travel. Ion species, thus, continuously collide with the counter flow drift gas, which causes them to slow down relative to each other depending on their collision cross-sectional area. As a result, different ions travel at different rates through the drift region. Longer drift regions provide more time for ions of different species to separate, which increases resolution and lowers signal intensity.
Performance features, such as resolution, signal-to-noise ratio, and durability, are greatly affected by the materials used in the construction of the drift tube, and the linearity of the electric field applied to propel the ions toward the detector. However, IMS instruments should be serviceable and easily maintained.
Gas-ion chemistry under ambient pressure can be complicated in an uncontrolled environment. Temperature, moisture, pressure, electric field, gas flow rates, drift gas composition, and ion density are factors in governing an ion's identity, drift time, peak intensity, and resolving power. Optimizing these parameters can be useful for the detection of compounds of interest.